How to delete part of an fbx model?
First, import the mesh by pressing File > Import > FBX and navigating to the .fbx file.



Once you have your object imported, ensure that it is the active object (orange outline). If it's not active, right click on it.

Then press Tab to enter Edit Mode where you can edit the underlying mesh.

In your case, you can select the parts of the mesh you want to delete by:


Selecting a vertex on the head with  RMB
Press CtrlL to select linked geometry
Press CtrlI to invert selection


Once you have the vertices you want to delete selected, press Delete > Vertices to delete them.